Abstract - Ecology of
transport is one of the most frequent topics of professional community
nowadays. Sustainable transport requires new solutions in vehicle design,
especially in their propulsion. For the design of public transport vehicles
drives it is important to work with parameters close to the real conditions.
This helps to find the best conception of the drive and so reduce the
production of air pollutants and the consumption of energy. Energy intensity of
the vehicle is usually determined on the basis of a driving simulation based on
driving cycles for measuring of harmful emissions and fuel consumption. These
simplified driving cycles do not include the grading resistance therefore they
are insufficient to determine certain specific conditions. Compilation of
driving simulation based on real driving cycles measured during the driving of
the vehicle in urban traffic seems to be more appropriate for specific
vehicles, such as buses for public transport are. This paper shows to what
extent the use of standard driving cycles for the design of hybrid drives,
respectively electric drives of vehicles is consistent with real conditions.

1. Introduction

According the Habarda [1] theigh efficiency
of vehicles drives, low level of harmful emissions and low level of noise are
basic requirements imposed on public transport vehicles. However the
conventional drives with internal combustion engines are no longer able to
successfully meet these requirements. The importance of the use of electric and
hybrid vehicles in city public transport therefore increase.

Chan et al. [2] considered transit buses to be the
best candidates for hybrid application because they normally operate on
predictable routes with frequent starts and stops. A hybrid propulsion system
is better suited for transit buses than private cars because they operate at
lower speed, limited acceleration, less road grades and ample space available
for batteries.

The information about intended use of some vehicles
and real condition at place of their operation are necessary to properly design
the drive of this vehicle. Efficiency of utilization of electric and hybrid
drive systems depends on their right dimensioning and on the appropriate
combination of their subsystems [3].

In order to reduce the vehicle emissions and
energy consumption, some systems for the accumulation of kinetic energy from
deceleration and braking which would be otherwise wasted by heat as well as
systems which switch off the combustion engine while vehicle standing, as
stop/start system, are currently designed. Proposals of similar devices as well
as the hybrid drives are usually based on driving cycles for determining of
harmful emissions and fuel consumption, such as Braunschweig bus cycle,
Millbrook Westminster London Bus cycle - inner London or BP bus cycle [4]. These
cycles are however simplified. They don't contain the grade resistance or real
progress of braking deceleration. In the normal traffic, there is often slow
down, respectively a short-term braking, which can be only a minor source of
recovered energy, as mentioned Huilong et al. [5]. For the purpose of
comparison of standardized and real driving cycles several measurements of real
driving cycles in different cities of Slovakia, Poland and Czech Republic.

As Gao et al. [6] introduced the dynamic
interactions among various components and the multidisciplinary nature make it
difficult to analyze a newly designed hybrid electric vehicle. Each of the
design parameters must be carefully chosen for better fuel economy, enhanced
safety, exceptional drivability, and a competitive dynamic performance-all at a
price acceptable to the consumer market. Prototyping and testing each design
combination is cumbersome, expensive, and time consuming. Modeling and
simulation are indispensable for concept evaluation, prototyping, and analysis
of Hybrid electric vehicle. Therefore the simulation of hybrid drive
of the bus based on the values from real driving cycles was performed.

The right timing of the measurement of real
driving cycles is very important. As stated in [7] there are big differences in
the traffic during the day and during the week too.

The chart on the Figure 2 shows how the number of
trips in progress changes over the course of 24 hours for different days of the
week. The chart is presented as an index, which compares the number of trips in
progress per hour on a weekday, a Saturday and a Sunday with the average number
of trips in progress per hour across all hours in a week.

Figure 2. Trips in
progress by time of day and day of week - index: Great Britain, 2012, [7].

The chart shows two peak hours for the number of
trips in progress on a weekday (Monday to Friday). The first (and highest peak)
is in the morning between 8:00 and 8:59 when there are nearly three times as
many trips in progress when compared with the average hour. The second peak is
in the afternoon between 15:00 and 15:59. Both peaks are driven by education
related trips. There is only one peak hour for trips in progress at the
weekend. On Saturday the peak hour is between 11:00 and 11:59 and on Sunday the
peak hour is between 12:00 and 12:59. Overall the number of trips in progress
on a Sunday is 24% lower when compared with the average day. The busiest day of
the week in terms of the number of trips was Friday with 150 trips per person
per year.

Based on these facts and because it is assumed
that electric and hybrid vehicles are used mostly for transportation to job and
shopping by their owners in middle European region, the measurements were
performed in real time of their use, in the time of afternoon rush hours from
14:00 to 16:30 hrs. This time, typical for our region, is characterized by
growing number of traffic congestions and longer standing at intersections. The
most energy-demanding regimes of frequent acceleration and braking were
recorded by this way.

One of the most varying factors affecting the
value of required vehicle performance is the elevation profile of the city.
Selected elevation profiles recorded during the measurement in Slovakia, Poland
and Czech Republic show the Figures 3 to 5.

In Zilina, there were measured driving cycles of
three bus lines (No. 21, 27 and 31) of the Zilina public transport. Table 1
shows the basic characteristic parameters of these driving cycles.

Figure 6 represents the elevation profiles of the
line routes 27 and 31. Results of driving cycles analysis shown relatively big
height differences in individual elevation profiles. Significant differences in
these values were recorded also between individual lines within the city. The
real driving cycles of specific bus lines were measured in both directions,
back and forth, to ensure the objectivity of measurements.

Figure 6. Elevation
profile of the bus line 27 and 31.
Figure 7. Percentage of
standing time in the real driving cycle.

Analysis
of the standing time (defined as a time when the speed and the acceleration of
the vehicle is equal to zero - bus stops, intersections, in traffic jam etc.)
useful for the stop/start system design comparing the standing time of measured
lines with Braunschweig bus cycle is shown in the Figure 7. As can be seen, the
measured values oscillate around the value of Braunschweig bus cycle.

3. Simulation of the hybrid bus drive

According to a study by Rahman et al. [8] among
many different hybrid configurations, the two generally accepted
classifications are series and parallel. The parallel hybrid is more efficient
satisfying high road power demands (high speed or acceleration), while series
is more efficient for large energy demands. For public transport purposes,
where large weight is of more concern than high acceleration, the series
propulsion strategy becomes more efficient and simpler. On the basis of that the
serial hybrid drive of a bus was chosen as a subject of the simulation. The
simulation was done in Matlab Simulink for 3 categories of buses (minibus, 10m
long bus and 12m long bus) with specific parameters for different occupancy 25,
50, 100% [9]. Measured actual driving cycles were used as an input to the
simulation. Required power of the bus was calculated based on selected
parameters and the real driving cycle.

Four variants of serial hybrid drive with
different parameters of key components were simulated. Two types of Li-ion
batteries with different capacity, two types of internal combustion engines
with different power, electric traction motor and two types of generators were
used [10]. Behaviour of individual components of the hybrid system was
simulated based on the control strategy. Internal combustion engine was
operated in one revolution mode with the best specific fuel consumption [11], [12].
The operation of the combustion engine was controlled by state of charge (SOC).
If the SOC reached preset lower limit the combustion engine was turned on. When
SOC reached preset top limit where the battery was fully charged, the
combustion engine was turned off [13].

The results of simulations based on the values
from real measured driving cycles can be seen on next figures. Analysed were
the recoverable energy from the driving cycle Erek, energy E and average power
Paver required to pass the cycle.

As Figure 8 shows, energy required to pass the
cycle E and the value of recoverable energy Erek increase with the occupancy of
the vehicle. There are some differences between values obtained at measurement
in the direction back and forth caused by changes in the number of descents and
climbs on the route (see elevation profile on Figure 6). This fact leads to the
need to take into account the measured values from both directions of the bus
line route, when design the bus drive.

The Figure 9 shows, that the maximum difference in
the values of average power at comparison with all three lines in both directions
represent for example at the 12m long bus with the weight of 12855kg up to
25kW, comparing the lines 21 and 27. This value represents 78% of the average
power needed for the passing the line 21.

Figure 8. Course of
energies - Line 21.
Figure 9. Course of
average power depend on the bus mass - 50% occupation.

The course of the energy which could be re-used
from the driving is represented by the Figure 10. As show these pictures, the
amount of recoverable energy depends on the various parameters of the bus line
route such as profile, distance and amount of decelerations. Due to the similar
elevation profile, number of stops and common part of the route the amount of
recoverable energy can be very similar for some city lines (line 27 and 31).
But it can be also very different for line routes led in different conditions
(line 21).

Figure 10. Course of
recoverable energy - 50% occupation.

The same situation is with the energetic
requirements on the buses. The energetic requirements on the bus drives can be
very diverse in one city. These results point to the need of individual
approach to the proposal of hybrid buses drives for each bus line.

The fact that the route and timetable of drive
throughout the day is known is big advantage at the design of public transport
buses. With this knowledge it is able to configure the hybrid drive to operate
as efficiently as possible. One of the assumptions is that the bus will start from
the depot with fully charged battery and when return to the depot the SOC is
low as possible so that the bus can be charged in depot.

In this case the battery will be charged from the
public electricity network and so it is possible to reduce production of
emissions that would be otherwise created by combustion engine during battery
charging. Second key factor is dimensioning of combustion engine size. As shows
the simulation the battery charging time depends on the size of the internal
combustion engine and battery capacity (Figures 11-14). Combustion engine in
the simulation run in optimum conditions with the lowest specific fuel consumption.
Engine power should be never less than the average power from the real driving
cycle because in this case cannot be guaranteed that the hybrid drive has
sufficient power for drive. Especially in city centres is big emphasis on the
production of harmful emissions. Therefore, it is important to propose the
hybrid drive and charging management so that the batteries will be charged
enough before entering into selected area of the city and will have sufficient
capacity for driving only on electric drive.

4. Conclusion

Correct design of a hybrid or electric drive
depends on the appropriate choice and arrangement of individual elements in
terms of their size, performance and capacity. Driving cycles measured in real
traffic capture specific driving conditions of the city, respectively specific
line characterized by the number of starts and stops, grade or driving speed
and better characterize the real situation of the traffic in the city. Impact
of the organization and the management of the traffic in the city greatly
affect the ratio of the time of standing and driving.

The concept of design of a modular hybrid drive
with adaptive management has its justification in the design of urban buses,
because it allows complete individualization of buses with hybrid drive with an
emphasis on ensuring minimum impact on the environment and reducing of economic
costs.

Acknowledgements

This paper is supported by following projects:

University Science Park of the University of
Zilina (ITMS: 26220220184) supported by the Research&Development
Operational Program funded by the European Regional Development Fund and
projects ITMS 26110230107, ITMS 26110230117, VEGA 1/0927/15 Research of the use of alternative fuels and hybrid drives on traction vehicles with aim to reduce fuel consumption and air pollutants production.